Laser Deicing Theoritical And Experimental Research For Transmission Lines

Ice deposited on overhead transmission lines, power network insulators, andhigh-voltage towers threatens the security of power utilities. About30types of de-icing andanti-icing techniques have been developed, but several methods are being used in practice.Laser technology, which has the advantages of high energy, non-contact operation, longdistance and high transmission efficiency, can be one of the novel and potential methods ofde-icing.The main research of this thesis was laser deicing for transmission lines. The laserpenetrating length into the ice was decided by laser wavelength. So in this thesis theviewpoint that laser irradiating the ice could be devided into volume thermal source andplane thermal source was presented. According to the thought, the theorical model of planeand volume thermal source irradiating ice was established. The numerical simulation andexperimental research of plane and volume thermal source irradiating the ice were alsoexplored. The melting ice rate and melting ice efficiency were obtained. The researchresults were as following:Firstly, basing on thermal conduction equation, according to the ratio of ice depth topenetrating depth, the judaging standard of the laser thermal type irradiating the ice wasobtained: when the ratio is more than6, the laser can be considered as plane thermal sourceand the temperature distribution could be obtained by1-D thermal conduction equation;while when the ratio is less than6, the laser can be considered as volume thermal sourceand the temperature distribution should be obtained by2-D thermal conduction equation.Secondly, when the laser with wavelength1.064micron irradiated the transparent andhard ice with depth less than258mm, the laser could be considered as volume thermalsource. The melting rate was influenced by laser power density, and most parts were heatedand the remaining ice became opaque and its structure became loose, apparently as a resultof thermal stress. A mechanical force was applied to the ice after laser irradiation in thedirection of the thermal stress and the remaining ice could be easily removed. This wouldspeed up laser deicing. The experiment was performed with a275-W pulsed Nd:YAG laserwith beam diameter38mm. The results were: the melted volume of ice per second was236.5mm³, and the laser energy consumed for melting1mm³of ice was1.33J. The maxim melted length of ice per unit time can reach2.123mm with270-W/4mm laser irradiated theice.Thirdly, when the laser with wavelength10.6micron irradiated the transparent andhard ice with depth more than38micron, the laser could be considered as plane thermalsource. The melting ice rate and melting ice efficiency are both influenced by laser power.Only the surface of the ice was heated, and the ice was removed layer by layer. Most of theremaining ice was still transparent and hard, as there was hardly any thermal stress. Theexperiment was performed with a CW CO₂laser with output power2000W, beam area693mm2. The results were1639mm³/s,1.221J/mm³and1.318mm/s.Fourthly, the theorical and experimental results showed that the melting ice rates andefficiencies of volume and plane thermal source were almost equal at the same powerdensity, and the irradiated ice was opaque due to the thermal stress. In principle, lasers withthe following characteristics could be selected for a power-line laser deicing system: thelaser has large penetrating length into the ice, and according to the ice depth whichdeposited to the transmission lines, the Nd:YAG laser is more appropriate for a laserdeicing system than CO₂laser. For the same laser, higher power or energy and larger beamspot under the condition of the same power density should be chosen for obtaining highermelting rate and efficiency.Finally, the research of high power laser irradiating insulator and aluminum line wasexplored. When the surface temperature of porcelain insulator reached250℃, the thermaldamage occurred. The thermal stress leaded to the breakdown damage. The experimentaldamage threshold of porcelain insulator was about11.1J/mm2. The compound insulator waseasily buring with laser irradiation. Aluminum lines were not damaged after high powerlaser irradiated for a long time.